Quantitative theories for the effects of age-specific breast cancer risk of ages at menarche, first childbirth and menopause have been proposed by Pike et al (32) and Moolgavkar et al (33,34). We have fitted the incidence rate functions predicted by these theories to data from our large case-control study of breast cancer in the San Francisco Bay Area (17). The inadequate fit of the functions to the data led us to develop a descriptive function with enough adjustable parameters to summarize adequately the combined effect of the above factors (35). Major findings include 1) the protective effects of late menarche and of early first full- term pregnancy (FFTP) are greater in premenopausal than in postmenopausal women; 2) incidence rates increase with age more rapidly before FFTP than after, but FFTP itself boosts the level of risk; 3) among the ever-parous, multiparity is protective both in premenopausal and postmenopausal women, regardless of age at FFTP; 4) both nulliparous and lean women are more protected by early menopause than are parous and overweight women; 5) increased body mass index is protective before and deleterious after menopause; and 6) postmenopausal incidence rates increase with age more rapidly among overweight than among lean women. The present project will seek to duplicate these findings by fitting the above incidence rate functions to data on 4400 breast cancer cases and 13,000 controls collected in seven countries as part of the International Collaborative Study of Breast Cancer. We will look for similarities and differences in findings across countries with high and low breast cancer rates. Findings that are consistent across all areas will be used to develop new leads to the etiology of breast cancer, and to profile women at high risk for the disease. The project is particularly cost-effective in eliciting fruitful information from a valuable existing data set, at a fraction of the cost incurred in collecting new data.